THE SENSE OF SIGHT. 



617 



by changes in the distinctness of images. Coincident with the 

 lens change, the pupillary margin advances and the pupil itself 

 becomes smaller. By this means an indistinctness of the image is 

 prevented by cutting off the rays which would give rise, owing to 

 the angle at which they fall on the surface, to diffusion circles, from 

 spheric aberration. 



The Function of the Ciliary Muscle. Though it is generally 

 admitted that the increase in the convexity of the lens is caused by 

 the contraction of the ciliary muscle, the exact manner in which this 

 is accomplished is not clearly understood. According to Helmholtz, 

 when the eye is in repose and directed to a distant object the lens is 

 somewhat flattened from a traction exerted by the suspensory liga- 

 ment. When the eye is directed to a near object, the ciliary muscle 

 contracts, thereby relaxing the ligament, as a result of which the lens, 

 by virtue of an inherent elasticity, bulges forward and becomes more 

 convex. In consequence of this latter fact the refracting power is 



FIG. 293. THE LEFT HALF REPRESENTS THE EYE IN A STATE OF REST. 

 RIGHT HALF IN STATE OF ACCOMMODAION. 



THE 



proportionally increased. In extreme efforts of accommodation it is 

 believed by some observers that the circularly arranged fibers, the 

 so-called annular muscle, contract and exert a pressure on the periph- 

 ery of the lens and thus aid other mechanisms in relaxing the ligament 

 and in increasing the convexity. This view appears to be supported 

 by the fact that in hypermetropia, where a constant effort is re- 

 quired to obtain a distinct image of even distant objects, the annular 

 muscle becomes very much hypertrophied, thus reinforcing the 

 meridional fibers. In myopia, on the contrary, where the accommo- 

 dative effort is at a minimum, the entire muscle possesses less than 

 its average size and development. 



According to Tscherning, a different explanation of the action of 

 the ciliary muscle must be given. Thus, when it contracts, the antero- 

 internal angle, that portion in close relation with the suspensory 

 ligament, recedes and exerts on the ligament a pressure which in 

 turn exerts a traction on the peripheral portions of the anterior 

 surface of the lens, which produces the deformation observed. 



